The present invention relates to a method for the preparation of an amorphous semiconductor film, and more particularly to a method of depositing a fluorinated amorphous silicon thin film.
Research has been proposed for practical applications of an amorphous silicon (hereinafter abbreviated as a-Si) film as a material for optoelectric energy conversion devices. This is because there is associated with the favorable property of a large coefficient of optical absorption as compared to crystalline silicon films. For example, the coefficient of optical absorption of an a-Si film is larger by about one decimal order than that of a mono-crystalline silicon film at the proximity of the peak of the solar spectrum. Therefore, even if the film thickness is reduced, the a-Si film can achieve an acceptable photo-electric energy conversion. Moreover, various substrates such as a metal substrate, a glass substrate, etc. can be chosen, unlike the substrate for a crystalline silicon film, as a substrate for depositing a-Si film thereon, whereas the substrate for a crystalline silicon film is limited to a crystalline silicon substrate.
In view of the aforementioned merits, solar cells using amorphous silicon are possible to reduce their material cost by about two decimal orders, as compared with the crystalling silicon solar cells. In addition, owing to the fact that a-Si films have a large coefficient of optical absorption, their applications to an image sensor in a facsimile apparatus and to thin film transistors for driving a liquid crystal display, are now being studied.
Although the a-Si film has excellent properties as described above, the film has some disadvantages caused by the existence of many dangling bonds therein. It is impossible to form a PN-junction and to control electric conductivity by doping impurities into the film because of the dangling bonds. However, an a-Si film prepared by the glow discharge decomposition process using a silane (SiH.sub.4) gas, has very few dangling bonds. This is surmised from the fact that hydrogen atoms trapped in the film compensate for the dangling bonds.
More particularly, the a-Si film prepared by the glow discharge decomposition of the silane gas contains about 10 to 40% hydrogen. The hydrogen ratio depends upon the preparation conditions. It is seen from the infrared absorption spectrum that the trapped hydrogen atoms form Si-H bonds. The hydrogen atoms forming such bonds are considered to serve as bonding partners for the dangling bonds. The hydrogen atoms compensate the dangling bonds to reduce a density of localized states in an energy gap to about 10.sup.16 cm.sup.-3 .multidot.eV.sup.-1. Thus, since the density of localized states is small, in amorphous silicon prepared by a glow discharge, the substitutive impurity doping is possible. Hence, the formation of a PN-junction as well as the control of an electric conductivity is possible.
However, the bonding energy of the Si-H bond is as small as about 3.5 eV. Hence, the dissociation of H from Si begins due to lattice vibrations which are caused by the heat above 350.degree. C. Furthermore, the dissociation of H from Si is also caused by the collision of carriers excited by intense light energy or carriers energized by the intense electric field applied to the lattice. Thus, the a-Si film including hydrogen is poor in the thermal stability, optical stability and electric field stability. Consequently, the above-described devices using a-Si film containing hydrogen as a film material have low reliability, and no guarantee of long life.
It has been proposed that the dangling bonds be compensated by fluorine which has a bonding energy (about 5.6 eV) that is larger than the bonding energy of hydrogen. The resulting a-Si film would have enhanced reliability and thermal stability.
However, as will be apparent from a description below, an a-Si film cannot be formed by the glow discharge decomposition method, employing only a silicon tetrafluoride gas. This face it unavoidable in the glow discharge decomposition method. In order to produce an a-Si film containing fluorine by the glow discharge decomposition method, it is essentially necessary to use a silane gas or a hydrogen gas in addition to silicon tetrafluoride gas. As a result, hydrogen cannot be excluded from an a-Si film. Therefore, the improvement in the reliability of devices employing a-Si films produced by the glow discharge method is not expected.
A method for producing an a-Si film not containing hydrogen at all has also been proposed. This proposed method employs a high frequency, two-electrode, sputtering apparatus. A silicon target is sputtered in a mixed gas atmosphere containing a silicon tetrafluoride gas and an argon gas. The sputtering deposits an a-Si film on a substrate. At the present time, in order to sputter the silicon target, it is necessary to make the proportion of the argon gas 90% or more of the mixed gas. As a result, 2-6% argon is contained in the deposited a-Si film. If argon is contained in the film, structural defects result. Moreover, due to the sputtering of a-Si film many additional structural defects would be produced in the a-Si film. For this reason, although hydrogen free a-Si film can be produced by the sputtering method, the prepared a-Si film is inferior to the film produced by the glow discharge decomposition method in the electrical properties such as a photo-conductivity and the like due to the structural defects.